Wet clutch friction systems are widely used in the field of automatic transmissions and transfer cases. Most clutches use several friction discs interleaved with several steel discs to provide torque transfer upon frictional engagement. This type of configuration is also used in braking applications. The frictional engagement is achieved through a piston disposed in a housing to enforce the frictional engagement between the friction discs and steel discs. The piston travels linearly in a cavity in a housing between an engaged (e.g. when pressure is applied) and a disengaged position (e.g. when the pressure is released and the return springs move the piston back). The amount of travel between the engaged and disengaged positions is often termed the pack clearance or “running” clearance.
When the pack is not engaged, the clearance allows for spacing between the friction disc elements interleaved with the steel disc elements. This serves two purposes, the first is to allow the friction disc elements to rotate independently from the steel disc elements, and the second is for the reduction of spin losses within the transmission. When there is not enough clearance between the separate elements, spin losses can result in drag torque. This reduces the efficiency of the transmission (or transfer case), and can also affect fuel economy. The pack clearance also has an effect on engagement time; the piston must travel the length of the pack clearance before the pack is fully engaged and full torque transfer occurs. A large pack clearance will lead to a longer engagement time, which can also affect the performance of the transmission or transfer case, depending on where the clutch is used. As transmissions and transfer cases improve in technology, reduced shift times are considered desirable.
One problem that occurs often in the manufacturing process is the variability of the thickness that can occur in the friction discs and steel discs that are used in a clutch pack. These variations in thickness in both of these elements can cause variations in running clearance once the clutch pack is assembled, affecting shift time, and spin loss characteristics. Typically, pack clearance adjustment is achieved through the use of shims. The shims take up extra space in the clutch pack so the apply piston travels a controlled distance to engage the pack.